1. Field of the Invention
The invention is concerned with the counterbalancing of vehicles that are capable of lifting, tilting or shifting a load relative to the vehicle frame and, more particularly, to an overload protecting or load limiting device for use on such a vehicle to prevent overturning of the vehicle during movement of the load.
2. Prior Art
While the present disclosure is directed primarily to load limiting devices for lift trucks it is to be understood that lift trucks are only representative of a general class of counterbalance vehicles which also include front end loaders, back hoes, tree diggers, cranes, two trucks and the like.
A lift truck is used to move heavy loads from one place to another. It includes a vertically extending primary load member commonly referred to as a mast which is pivotally connected to a frame of the truck. Generally, the truck rides upon wheels in a normal manner. A hydraulic piston-cylinder device is generally used to tilt the mast relative to the frame about a pivot point. A secondary load member, for example a pair of forks, are movable along the mast and in certain applications are capable of tranversing the mast. The truck can thus pick up, transport, elevate, lower, shift and/or tilt a load for placement purposes.
It has been a major problem of the prior art in operating such a vehicle that the vehicle tends to tilt about one of the wheel axles thus causing the vehicle to overturn. An overturn condition comes into existence, of course, when the center of gravity of the lift truck, including the load, falls either forwardly of the front axle of the truck or rearwardly of the rear axle of the truck. This condition can be caused by any one of a number of factors. For example, the size of the load, the distance of the center of gravity of the mast, forks and load from the respective front or rear axle, dynamic forces created by acceleration and deceleration of the truck, dynamic forces created during elevation, lowering and tilting of the mast and forks and the like. It is also possible for lift trucks to be overturned sideways about the front and rear wheels on either side of the lift truck. In sidewise overturning the dynamic forces of shifting a load sideways must be considered as well as the distance of the composite center of gravity of the mast, sideways extending forks and load from the wheels of the truck.
A great number of attempts have been made to solve the overturning problem in lift trucks. A discussion of many of these attempts may be found in U.S. Pat. No. 3,831,492 to Young. The Young patent itself teaches an overload protection device which includes a fluid activated double-acting piston-cylinder and a pressure-actuated valve which operates in response to a pressure signal respresentative of the difference in pressure on opposite sides of the piston to prevent movement of the load members in one direction when the overturning moment on the vehicle exceeds a safe value. The device of the Young patent is useful in accomplishing overload protection, but is relatively complicated in structure. U.S. Pat. No. 3,866,419 to Paul discloses an integrated pressure-compensated load sensing system for a lift truck or the like which, again, is rather complicated in structure U.S. Pat. No. 3,307,656 to Susag discloses a relief valve means for preventing overturning of a lift truck.
The prior art devices for preventing overturning of a lift truck or the like suffer from any one or more of the following disadvantages: they fail to permit certain needed operations of the lift truck when a non-dangerous overload condition exists, e.g. increasing tilt or lowering of the load; they are too expensive; they are difficult to install; they reduce the efficiency of the vehicle; they operate in a manner which can cause overturning of the vehicle; they do not detect the overturning moment created by the load; they cannot be easily fitted on to existing vehicles; they create false overload signals causing intermittent operation of the vehicle when an overload condition does not exist; they lack flexibility for readily adapting them to a wide range of vehicles; and they are so complicated that they provide serious maintenance and adjustment problems.
It is accordingly a primary object of the invention to provide a relatively compact load-limiting device for use in a load limiting apparatus, which device has none of the abovementioned drawbacks.